The nonlinear optical effect originates from the interaction between the laser and the medium. When the laser propagates in a medium with a non-zero second order polarizability, the nonlinear optical effects such as frequency doubling, sum frequency, difference frequency and optical parametric amplification will be produced. Based on the second order nonlinear optical effects, the crystals can be made as second harmonic generator, frequency converter, optical parametric oscillator and nonlinear optical devices, etc. and have important application in many fields, such as laser technology, atmospheric monitoring, national defense and military, etc. Inorganic nonlinear optical materials are dominant in the practical research of the second order nonlinear optical materials. According to the transmittance and application wavelength, the inorganic nonlinear optical crystal materials can be divided into ultraviolet nonlinear optical material, visible nonlinear optical material and infrared nonlinear optical material. The practically applied inorganic nonlinear optical materials in ultraviolet and visible light utility are BBO (β-BaB2O4), LBO (LiB3O5), KDP (KH2PO4), KTP (KTiOPO4), etc., they can meet the basic requirements of the most situation. However, for the infrared nonlinear optical materials, there is still far from practicality. The reason are as follows: although the existing infrared nonlinear optical materials, such as AgGaS2, AgGaSe2, has large second order nonlinear optical coefficient and have a wide transmittance ranges in the infrared region, the harsh reaction conditions, difficult to grow single crystals with high quality, especially the low damage threshold, result in the unsuitable utilities in nonlinear optical crystal materials. The realization of the frequency conversion in infrared region is of great value in the national economy, national defense and other fields. In 2003, the book titled as “nonlinear optical crystal materials science” published by Science Press (China) proposed that “Infrared nonlinear optical crystal is underdeveloped in the whole spectral band of nonlinear optics, therefore, the researches of novel nonlinear optical materials in infrared needs to be strengthened”. Hence, it is a difficult and hot spot to find new high efficiency infrared nonlinear optical materials with high laser damage threshold and large nonlinear optical coefficient. In 2007, O. I. Siidra, et al reported the Pb2.16OCl2.32 with the space group of Fd2d and the cell parameter of a=5.829(3) Å, b=16.056(6) Å, c=35.528(15) Å (Doklady Physical Chemistry, 2007, Vol. 414, Part 2, pp. 128-131.). The space group and the cell parameters of the compound are different from that described in the present invention. In 2014, invention patent titled as “compounds and lead oxychloride infrared nonlinear optical crystal and preparation method and use thereof” (Patent No.: ZL 201410626434.2) was authorized by Xinjiang Academy of Sciences Institute, Chinese Academy of Sciences. Therein, the invention reported Pb2.14OCl2.28 compounds. the present invention was further systematic research based on Pb2.14OCl2.28 and the obtained infrared nonlinear optical materials have a general formula of Pb2+xOCl2+2x, therein 0<x<0.139 or 0.141<x<0.159 or 0.161<x≤0.6. Because of the obvious evaporation will lead to the component deviation in Pb—O—Cl system during the crystal growth, the optical crystal with general formula Pb2+xOCl2+2x presented in this invention is more universal and practical than Pb2.14OCl2.28.